Array antenna system and algorithm applicable to RFID readers

ABSTRACT

Embodiments of the invention pertain to Radio Frequency Identification (RFID) method and system using an antenna array, an array controller, and control algorithms. Embodiments of the invention can induce strong radio-frequency (RF) excitation, for a given level of radiated RF power, at any point within an arbitrary inhomogeneous medium. For RFID applications, one typical inhomogeneous medium is an ensemble of cases on a pallet. Another typical medium is a warehouse environment having stored goods together with shelving and other material present. An embodiment of the invention is applicable to the process of reading battery-less, or “passive” RFID tags, which rely on incident RF electromagnetic fields established by RFID readers to power the electronic circuitry within the tags.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/618,779, filed Feb. 10, 2015; which is a continuationapplication of U.S. application Ser. No. 14/085,294, filed Nov. 20,2013, now U.S. Pat. No. 8,952,815; which is a continuation of U.S.patent application Ser. No. 12/363,555, filed Jan. 30, 2009, now U.S.Pat. No. 8,593,283; which claims the benefit of U.S. ProvisionalApplication Ser. No. 61/062,998, filed Jan. 30, 2008; all of which arehereby incorporated by reference herein in their entirety, including anyfigures, tables, or drawings.

BACKGROUND OF INVENTION

Battery-less tags, by virtue of their potentially ultra-low cost andessentially unlimited shelf life, are important components for a broadclass of important RFID applications. When an RFID inventory-trackingscheme requires every case or item within the purview of aninventory-control system to be tagged, which is the typical case forretail-distribution applications, battery-powered tags are generallyconsidered cost-prohibitive, and battery-less tags are most often theonly viable choice. When long-term storage of tagged items is involved,such as in a physical records archive managed with RFID technology, thefinite shelf-life of batteries is an additional strong motivator for theuse of battery-less tags.

Despite significant advances made in recent years in battery-less RFIDtag technology, the adoption of this technology has significantly laggedthe original expectations for RFID technology. An important impedimentto more widespread adoption and utilization of battery-less RFIDtechnology is the poor performance that is still frequently experiencedwhen tags are on or near items that contain or comprise materials thatinteract strongly with RF propagation. Such materials include metal,dielectrics and lossy dielectrics that reflect, refract or attenuate RFenergy incident on them or passing through them. Cans, foils, liquids,gels, dense powders, produce, meat and dairy products are just a fewexamples among numerous items that can severely impair the RF couplingbetween a reader and a tag.

Severe attenuation of a signal propagating from an RFID reader to abattery-less RFID tag is particularly problematic. The RFelectromagnetic field strength required to operate a battery-less RFIDtag is significantly higher than that required to communicate to anelectronic receiver having an independent power supply such as abattery. Active electronic circuitry, powered by a battery or otherpower source, can indeed detect, decode and otherwise process extremelyweak signals. A battery-less RFID tag, however, cannot operate suchelectronic circuitry until the tag has extracted sufficient energy fromthe RF electromagnetic field supplied by the reader or another externalsource. The incident RF field level required to provide operating powerfor the electronic circuitry is far greater than that required tocommunicate with already-powered circuits. The frequent difficulty inachieving the necessary incident RF field strength in the presence ofmaterial configurations with adverse RF propagation characteristics,while still satisfying regulatory constraints on radiated RF powerlevels, is an important technical obstacle currently preventing far morewidespread adoption of battery-less RFID technology.

In prior applications of antenna arrays, there are typically only one ortwo degrees of freedom exploited, corresponding to elevation and azimuthangles for the far-field radiation pattern. In relatively rareapplications, multiple beams might be formed, or radiation might befocused at a finite distance rather than at infinity, whereas far-fieldpatterns are essentially “focused at infinity”. Even in such relativelyexotic applications, however, the degrees of freedom utilized are farless than the total degrees of freedom inherently available withindependent control of individual antenna elements.

Prior applications of array technology are characterized by one or moreof the following:

-   -   The medium is homogeneous, such as free space, or sufficiently        close to homogeneous such that a homogeneous medium is assumed        for the control of the array;    -   The medium differs from a homogeneous one by a constant, known        factor, such as a protective radome, a supporting structure that        interacts with the array, a ground plane or approximate ground        plane, or a nearby half-space filled with a different        homogeneous or approximately homogeneous medium;    -   The medium is sufficiently inhomogeneous to affect the        propagation in a potentially adverse way, as exemplified by an        environment containing walls, trees or other structures, but the        antenna system makes no adjustments specific to the particular        configuration of this surrounding material, other than possibly        an adjustment in its angular sensitivity, e.g., pointing        direction;    -   The array does make adjustments that mitigate the effects of        adverse propagation characteristics such as multipath, but        requires the presence of a signal originating from the intended        focal point in order to adapt the array settings, as is the case        for a “rake receiver,”    -   The array makes adjustments that peak its response to a signal        emanating from an unknown location, but requires the presence of        a signal emanating from that location, and particular to it        (different in some way from similar signals that may be        emanating from other locations), in order to adapt the array        settings, as in the case of adapting the array to a transponder        or modulator with a self-contained power source;    -   The array makes adjustments that mitigate for unwanted signals,        in which case there is by definition a signal originating from        the location or direction of the intended null in the array        antenna pattern.

BRIEF SUMMARY

Embodiments of the invention pertain to Radio Frequency Identification(RFID) method and system using an antenna array, an array controller,and control algorithms. Embodiments of the invention can induce strongradio-frequency (RF) excitation, for a given level of radiated RF power,at any point within an arbitrary inhomogeneous medium. For RFIDapplications, one typical inhomogeneous medium is an ensemble of caseson a pallet. Another typical medium is a warehouse environment havingstored goods together with shelving and other material present. Anembodiment of the invention is applicable to the process of readingbattery-less, or “passive” RFID tags, which rely on incident RFelectromagnetic fields established by RFID readers to power theelectronic circuitry within the tags.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a top-level schematic diagram of the antenna array, alongwith a representation of an RFID tag located within an ensemble of boxesaccording to embodiments of the subject invention.

FIG. 2 shows a two-element array, controlled with a network of phaseshifters and 90° hybrid couplers according to embodiments of the subjectinvention.

FIG. 3 shows an embodiment of the subject invention utilizing a fixed1:N power divider, and a two-state (0°/180°) phase shifter for eacharray element.

FIG. 4 shows an interface module which can be used to connect thesubject invention with existing readers to allow the subject inventionand existing readers to be compatible.

FIG. 5 shows a pair of RF amplifiers which can boost both the transmitand the receive signals according to embodiments of the subjectinvention.

FIG. 6 shows a reader controller which can accept externally-generatedinputs to specify the array state and to trigger the transition of thearray between states.

DETAILED DISCLOSURE

Embodiments of the invention can enable the achievement of sufficientincident RF field strength in the presence of material configurationswith adverse RF propagation characteristics, as well as enhance theperformance of other systems and devices. Embodiments of the inventionincorporate an array of antenna elements, with an array controller thatindependently controls the various transfer functions between a commonRF input/output port and the various antenna elements. Embodiments canalso utilize one or more algorithms for arriving at one or more suitablesets of transfer functions for the various element signal paths.Embodiments utilize control algorithms that do not require advanceknowledge of the propagation characteristics of the media surrounding agiven point of interest. Additional embodiments can utilize advanceknowledge of some measurable properties that depend on propagationcharacteristics to increase the effectiveness of the system.

FIG. 1 shows a top-level schematic diagram of an antenna array inaccordance with an embodiment of the invention, along with arepresentation of an RFID tag 2 located within an ensemble of boxes. Asingle RF input/output port 4 is coupled to multiple array channelsthrough a 1:N power divider. The 1: N power divider can also functionfor the opposite propagation direction (the “receive” direction) as anN:1 power combiner. Each of the enumerated ports is coupled through anelectronically controllable variable phase shifter (or variable delayline) 8 to its respective antenna element 10. The phase shift may alsobe in inherent function of the power divider network, or may be partlyderived from the inherent function of the power divider network. Thevarious antenna elements can be characterized by different phase-centerlocations, different radiation patterns, and/or different polarizations,or different combinations of these attributes.

The power divider can be fixed, or can be variable. In this way, theamplitudes A₁, A₂, A₃, . . . A_(n) indicated at each of the N enumeratedports can be fixed, or can be electronically controllable to achievevariable power distributions within the constraint of the totalavailable input power. The power divider can be nominally lossless, withthe total transmit power delivered to the antenna elements differingfrom the input power at the RF port only by the amount of powerdissipated in unintended insertion loss. Alternatively, the powerdivider can incorporate attenuation in its mechanism for setting theoutput amplitudes. The power distribution mechanism can also use aspace-feed configuration, wherein one illumination antenna conveys powerto the various array elements, which then apply their local controlfunctions and re-transmit the signal into to the volume of interest.

As an alternative to a power distribution function, various means can beused to reconstruct the input signal at each array element, includingdigital sampling and reconstruction. In the case of digital sampling andreconstruction, control functions such as phase and amplitude controlcan be accomplished numerically in the digital domain beforereconstructing the RF signal.

The amplitudes established for the transmitted signals at the variousantenna elements of the array are denoted by the A_(n), where amplitudeA_(n) is defined here as the square root of the power delivered to theantenna terminals of array element n. The phases of these transmittedsignals, relative to the input signal at the RF input/output port, aredenoted by the ϕ_(n).

Propagation channels from the various antenna elements to an RFID tag'santenna terminals are denoted in FIG. 1 with dashed lines. In general,for a tag in the vicinity of RF-interactive materials, the couplingbetween an antenna element and the tag is a complicated phenomenon.However, though it is very difficult to calculate or predictanalytically, the coupling at any given RF frequency can becharacterized by a pair of parameters, namely, the amplitude, denoted byC_(n), and phase angle, denoted by γ_(n), of the RF voltage developed atthe tag's antenna terminals when an excitation of unit voltage and zerophase angle is applied to the terminals of antenna element. Thus, eachpropagation channel indicated in FIG. 1 can be characterized at aparticular frequency by the complex expression, C_(n) exp{jγ_(n)}, whichrepresents the complex steady-state transfer function at that frequencybetween the array element antenna terminals and the RF tag antennaterminals.

The RF voltage, V, induced at the RFID tag antenna terminals for anarbitrary combination of array element amplitudes A_(n) and elementphases ϕ_(n), is given by

$\begin{matrix}\begin{matrix}{V = {{Re}\{ {\sum_{n = 1}^{N}{A_{n}\exp\{ {j\;\phi_{n}} \} C_{n}\exp\{ {j\;\gamma_{n}} \}}} \}}} \\{= {\sum_{n = 1}^{N}{A_{n}C_{n}{\cos( {\phi_{n} + \gamma_{n}} )}}}}\end{matrix} & (1)\end{matrix}$

The sum of the individual transmit powers transmitted to the arrayantenna element terminals can be constrained to equal some value P₀:Σ_(n=1) ^(N) A _(n) ² =P ₀  (2)

If attenuators are used as part of the amplitude-setting mechanism inthe power divider, the input power can be adjusted to overcome theattenuator loss and maintain the overall power constraint. In general,an approach using attenuators to set the amplitudes will not optimizethe induced voltage at the antenna terminals of an RFID tag for a givenarray input power applied to the RF port, but with adjustable arrayinput power an approach using attenuators can optimize the inducedvoltage for a given total transmitted power, P₀.

The phases, ϕ_(n), of the array elements have no effect on theconstraint of Equation (2). Therefore, for any given distribution ofarray amplitudes, A_(n), the tag voltage, V, can be maximized over allpossible ϕ_(n) without accounting for this constraint. A condition for alocal maximum or minimum of V over all possible ϕ_(n) is that thegradient of Equation (1) with respect to the ϕ_(n) be zero. Thisgradient is zero when sin(ϕ_(n)+γ_(n))=0 for all n. For each n, twounique solutions for that ϕ_(n) are possible, corresponding tocos(ϕ_(n)+γ_(n))=±1. Since the A_(n) and C_(n) are all positive, realvalues, the sum can be maximized by choosing cos(ϕ_(n)+γ_(n))=1 for alln. This yieldsϕ_(n)=−γ_(n) ,∀n  (3)For any set of array element amplitudes, A_(n), this condition on theelement phases yields the maximum possible terminal voltage, V, for thatset of amplitudes.

Given the condition of Equation (3) for the phases, the tag terminalvoltage of Equation (1) becomes

$\begin{matrix}{V = {\sum\limits_{n = 1}^{N}{A_{n}C_{n}}}} & (4)\end{matrix}$

The tag terminal voltage of Equation (4) can be optimized subject to theconstraint of Equation (2) using the method of Lagrange multipliers. Toapply this method, the gradient with respect to the A_(n) of P₀ as givenin Equation (2) is set equal to some constant, λ, times the gradient ofV as given in Equation (4). This yields2A _(n) =λC _(n) ,∀n  (5)Solving Equation (5) for A_(n) and substituting the result into theconstraint of Equation (2) yields

$\begin{matrix}{\lambda = {2\sqrt{\frac{P_{0}}{\sum_{n = 1}^{N}C_{n}^{2}}}}} & (6)\end{matrix}$

The optimum amplitudes, given the condition of Equation (3) on thephases, are then given by

$\begin{matrix}{A_{n} = {\sqrt{\frac{P_{0}}{\sum_{n = 1}^{N}C_{n}^{2}}}C_{n}}} & (7)\end{matrix}$

Accordingly, for any amplitude distribution established by the powerdivider, there exists a unique set of phases, ϕ_(n), that optimizes(maximizes) the voltage induced at the antenna terminals of a particularRFID tag within the field of view of the array. Furthermore, when theamplitudes of the power divider can be adjusted, subject to a constanttransmitted power constraint as in Equation (2), there exists a uniqueset of power-divider amplitudes that further optimizes this inducedvoltage.

The propagation channel characteristics given by the C_(n) and γ_(n) aretypically not known, so the array element weights defined by the A_(n)and ϕ_(n) are difficult to calculate, or cannot be calculated, inpractice. However, a rational search algorithm can be defined so thatthe antenna array state can achieve an optimal setting or a sufficientlyeffective setting, for any location and polarization within the readvolume. This approach is very amenable to the RFID application, as RFIDreaders typically execute numerous read cycles for a given population oftags, providing numerous opportunities to cycle through different arraystates in order to arrive at an effective array state for each tag inthe population.

The array state can be parameterized by the phase, and optimallyamplitude, of the transmitted signals at each antenna element. Ingeneral, when the individual antenna elements have fixedcharacteristics, and the array controller only adjusts phase, or phaseand amplitude, of the excitations for each element, an N-element arrayhas at most N−1 meaningful degrees of freedom for phase control. Thephase of any one element can be arbitrarily established, and theresulting electromagnetic field intensity is dependent only on therelative phase differences of the remaining N−1 elements. When amplitudecontrol is implemented, there are at most N−1 degrees of freedom foramplitude control if the power divider is nominally lossless, or if atotal-power constraint such as Equation (2) is used, or if the inputpower is adjusted for each array state to achieve some definedconstraint on field intensity. Combined phase-and-amplitude of all Nelements independently can yield 2N−2 total degrees of freedom.

In an embodiment, the maximum definable degrees of freedom are notimplemented in the RF hardware. In a specific embodiment, for example, afixed power divider with no amplitude control can be used. Anotherembodiment can utilize variable power division between subsections ofthe array, such as sub-arrays with fixed power division within eachsub-array. Furthermore, given the degrees of freedom that can beimplemented with a given embodiment of RF hardware, it is not requiredto actually exercise all of them. An example would be a system utilizingdigitally controlled phase shifters with B bits of control, inherentlycapable of 2^(B) states. In a specific embodiment of these 2^(B) states,only those approximating certain discrete phase angles, such as 0°, 120°and 240° might actually be utilized.

With respect to various embodiments of the invention, the subject methodand apparatus can be used in applications having one or more of thefollowing characteristics: the propagation characteristics of the mediumare very likely to be highly inhomogeneous; there is insufficientadvance knowledge of the details of the medium to design an adequatelyfunctional array excitation ahead of time; the inhomogeneous nature ofthe medium is highly likely to affect the propagation in a sufficientlyadverse manner as to cause the application to fail if the adverseeffects are not mitigated with a fairly high degree of specificity; noactive signal sources or modulators at the desired focal points areavailable to assist the adaptation of the array for those locations; andeffective utilization is made of array states that would be ineffectivefor many prior applications.

In specific embodiments, array states may be parameterized by the actualRF control-component settings rather than by the resultant phases orphases and amplitudes of the element excitations. The number of RFcontrol components may exceed the useful degrees of freedom of the arraystate, in which case linear combinations of component settings may beidentified to bring the degrees of freedom for component settings inline with the useful degrees of freedom for the array state. This can beillustrated with the simple example of a two-element array, controlledwith a network of phase shifters and 90° hybrid couplers as shown inFIG. 2. The amplitudes and phases of the two antenna ports are given by

$\begin{matrix}{{A_{1} = {\sin( \frac{\theta_{1} - \theta_{2}}{2} )}}{\phi_{1} = {\frac{{180{^\circ}} + \theta_{1} + \theta_{2}}{2} + \theta_{3}}}{A_{2} = {\cos( \frac{\theta_{1} - \theta_{2}}{2} )}}{\phi_{2} = \frac{{180{^\circ}} + \theta_{1} + \theta_{2}}{2}}} & (8)\end{matrix}$

For this simplified example there are only two useful degrees offreedom, namely, the amplitude split between the ports and the phasedifference between them. To define array state parameters in terms ofcomponent settings, a linear combination of θ₁ and θ₂ could be defined:Δθ=θ₁−θ₂  (9)The two component parameters, Δθ and θ₃, are sufficient to span theuseful degrees of freedom for the array states.

In an embodiment, linear combinations of parameters may also be used forother reasons, such as increased convenience in categorizing states, andpotentially eliminating some states so as to reduce the search space.For example, consider a rectangular array of antenna elements with N₁rows and N₂ columns, containing N₁×N₂ elements. With combined phase andamplitude control of these elements, there are 2×N₁×N₂−2 meaningfuldegrees of freedom. However, element excitations could be defined interms of spatial frequency across the array. At some point, higherspatial frequencies would yield field configurations more and moreevanescent in character, i.e., characterized by non-propagating,stored-energy fields with amplitudes decreasing exponentially withincreased distance from the array. While some evanescent fieldconfigurations will actually couple to useful propagating modes within ahighly inhomogeneous medium, the array states with the highest spatialfrequency of the illumination may be found for a given application toadd no real utility. Parameterizing the array states in this way wouldfacilitate the definition of a subspace of useful array states.

In a specific embodiment, digitally controlled components are utilizedto control the array state, such that discrete states in the parametersearch space are defined by available states of the components. Forexample, if phase shifters (or switched delay-lines) with two-bitdigital control are utilized, then each phase shifter has four availablestates. For N−1 phase values in the parameter search space(corresponding to N array elements), there would be 4^(N-1) uniquecombinations of phase commands for the N−1 phase shifters. The entireparameter state space in this case could be represented by a singledigital word with 2×(N−1) bits. Incrementing this word through all4^(N-1) possible digital values would effectively yield an exhaustivesearch, subject to the control component resolution, through the entireparameter search volume, necessarily passing through the best availablestate for every location and polarization within the read volume.

In further embodiments, some or all of the control components can becapable of continuously variable states, rather than discretely variablystates. For example, voltage-controlled phase shifters can be utilized.Such components can be controlled with analog voltages that are in turngenerated digitally, thus rendering discrete states. Alternatively,analog control voltages can be generated to sweep these componentsthrough their states. To accomplish this, minimum-shift intervals wouldbe defined within the continuum associated with each such component.Each minimum-shift interval can define a relatively small interval ofvalues to which a control device can be constrained as other controlcomponents are swept through their full range of values. That is, eachminimum-shift interval can approximate a constant component setting. Forexample, if voltage-controlled phase shifters are controlled withsawtooth voltage waveforms, then the slowest sawtooth waveform moves itsphase shifter through one minimum-shift interval while the next slowestsawtooth waveform moves through its full range. Similarly, thissecond-slowest sawtooth moves its phase shifter through only oneminimum-shift interval while the third-slowest sawtooth moves throughits entire range. In the time required for the slowest sawtooth waveformto pass through its full voltage range, the full set of N−1 sawtoothwaveforms can effectively execute a raster scan of the parameter searchvolume, with a resolution defined by the minimum-shift intervals,necessarily passing through the best available state for every locationand polarization within the read volume.

Stepping the array through every available state in a methodical fashionmay be a tractable approach, particularly in cases where there is alimited number of array elements and a limited number of control statesfor each element, or in cases where the time required to execute thesearch is not as critical as it may be in applications involving rapidmovement of goods. An example of the latter case might be an electronicinventory conducted of an entire warehouse, where one complete inventoryprocess accomplished in a single evening represents an enormousimprovement over manual inventory processes. For more time-criticalapplications, however, utilizing a relatively large number of arrayelements and/or a relatively large number of control bits for eachelement, it may be useful to modify the search process.

In an embodiment, the search states can be ordered so as to increase theprobability of satisfying the incident-field requirements for everylocation (e.g., for every tag) relatively early in the search process.If the search process can be terminated upon obtaining a specifiedresult, such preferential ordering of states can significantly speed theinventory process. In RFID applications, a list of tags that should befound in an ensemble is often available in a database accessible by thereader. The search process can be terminated once all tags in the listare found, thus eliminating the use of subsequent array states. In manyapplications this would provide an acceptable level of inventorycontrol. In another embodiment, the parameter search volume can berestricted in some way, so that the total set of possible array statesis not implemented in any search.

The parameters for modified search processes may be based on laboratorydata, data from actual operation in the field, computer simulation orother analyses, or a combination of these sources. Modified searchparameters may be set at the factory during manufacture of the arraysystem based on currently available data for the intended application.Alternatively, or in addition, modified search parameters may becontinuously or periodically updated based on data accumulated duringactual operation within the specific operating environment for a givenarray system, or possibly from similar operating environments within thesame enterprise or across a set of cooperative enterprises.

In an embodiment, the available states can be ordered by cycling throughcoarse states first, then increasingly fine states. For example, ifthree-bit phase shifters are utilized, a search pattern can beconstructed for one-bit phase shifters. A second search pattern caninclude all possible states with two-bit resolution, with any duplicatearray states, already encountered in the one-bit search pattern, deletedfrom the new pattern. Another search pattern through all possible stateswith three-bit resolution can then be performed, with states alreadyencountered in the one-bit and two-bit search patterns deleted from thenew search. Sequencing through all three such search patterns in this“staged resolution” search pattern can achieve all array statesavailable with three-bit resolution. However, for any given polarizationat any given point, the voltage amplitude induced with the bestavailable one-bit array state can be a significant percentage of thevoltage induced with the best available two-bit array state, which inturn can be an even higher percentage of the voltage induced with thebest available three-bit state. That is, there is a decreasing rate ofreturn obtained from the increasingly higher-resolution states. Thus,only the most difficult cases require the peak performance achieved inthe latter stages of the complete search. This increases the likelihoodthat a search can be terminated before cycling through all availablestates.

Another way to formulate the “staged resolution” search pattern justdescribed is to define a binary number with Nbits×(N−1) bits, whereNbits is the number of bits of resolution for one control device, and Nis the number of control elements. The lowest-order N−1 bits (capable ofrepresenting binary numbers from 0 to 2^(N-1)−1) represent themost-significant bit for each of the N−1 control devices (for example,the 180° bits for all the phase shifters). The next N−1 bits in thebinary number represent the next-most-significant bit for each controldevice (for example, the 90° bit for all the phase shifters), and so on.Starting this binary number at zero and incrementing it by ones untilall states are achieved results in the staged resolution search pattern.

A second method for ordering the search states is to use prior knowledgeof particularly effective array states to define a set of preferredarray states for a given application. In an RFID application, this priorknowledge may pertain to a specific ensemble of tags, to a relativelynarrow category of ensembles typical for a particular RFID application,or a wide variety of ensembles typical for a particular RFIDapplication. The knowledge may be based on a particular RFID readerinstallation with a particular array configuration and operationalalgorithm, or may be derived from data pooled from a number of RFIDreaders.

Knowledge of particularly effective array states for RFID applicationscan be acquired by simply counting the number of unique tagssuccessfully read with each array state as a read process progresses.Those states for which any tag was successfully read can simply be rankordered, in descending order according to the number of tagssuccessfully read. This data can either be used directly in that form toprioritize array states, or may be processed to give higher scores tostates that successfully read tags not read with other array states. Thedata can also be more intensively processed, so as to derive a minimalset of array states that would have achieved all of the successful readsbased on the available data.

Preferred array states for a given RFID application may be derived froma specific configuration of a particular ensemble of tags, such as thetags associated with a particular shipment of cases in a fixedconfiguration on a pallet. In this case, the preferred ordering ofstates can be stored in a database accessible to RFID readers that maysubsequently process the same ensemble configuration. If the identity ofthe ensemble is not already known by other means, database functions canbe utilized to associate a single item within an ensemble with theparticular ensemble within which it should be found, enabling databaseentries for the appropriate ensemble to be accessed. The preferredordering of states could be one of the data items associated with thatensemble.

Preferred array states for an RIFD application may pertain to arelatively narrow category of tag ensembles. Ensembles may becategorized by vendor, product type or packaging mode, specificcombinations of these or similar attributes, or by any categorizationdetermined to correlate with different effective array states. Given alist of items that should be contained within a particular ensemble, theproper category can be inferred using various database functions tooperate on the list.

Preferred array states for an RFID application may be defined for broadcategories of tag ensembles. For example, data may be accumulated fromall read processes over an extended period of time from a largepopulation of readers and processed to form preferred states based onthe entire aggregate of data. In this case the broad category includesall ensembles that have been processed by the readers involved over theextended period of time.

Preferred states for either narrow or broad categories of tag ensemblesmay be derived from multiple array antenna systems with similar elementconfigurations, or from a single array system. Without recourse to datafrom any database or other reader, the controller associated with asingle array system could maintain a running total of the number ofsuccessful reads for each available array state, and a correspondingordered list of preferred array states. With access to a databasecontaining information about specific items associated with the tags itreads, the controller associated with a single array can also developand maintain separate lists of preferred array states for variouscategories of tag ensembles. Alternatively, read performance as afunction of array state can be compiled for a number of similar arraysystems in similar RFID applications, and the aggregate data processedfor preferred states.

For a particular RFID application or category of applications, it may befound that some array states are essentially unnecessary, in that theyrarely or never provide effective excitations that are not achieved byother states. For example, depending on array element spacing andlocation, certain combinations of element phases may yield highlyevanescent fields that essentially die off before illuminating anyuseful portion of the read volume. In the ordered search approachdiscussed above, such states would necessarily be at the very end of thelist. In more time-critical applications, they can be eliminatedaltogether from the search list, and other remedies arranged forsituations where one or more expected tags were not found while usingthe preferred array states on the list.

As another possibility, it may be determined for certain RFIDapplications that the performance of a certain subset of array elementscompliments the performance of another subset of elements, so that thesimultaneous combination of the two subsets is not necessary to achievethe required overall performance. An example of this situation might beobserved with the division of the overall array into multiplesub-arrays. It may be determined that with sufficiently high probabilityany tag encountered in the application will be successfully read by onesub-array or another operating by itself, in which case the exhaustiveset of all possible combinations of simultaneous settings for themultiple sub-arrays would not be required. Rather, each sub-array insuch a set would be activated in isolation, with the other sub-arraysset to zero amplitude, thus substantially reducing the dimensionality ofthe parameter search volume. For example, individual sub-arrays placedon each side of the inventory volume, or on each side and above thevolume, might be operated in this fashion.

Yet another situation in which the parameter search volume may berestricted would arise when the illumination patterns of differentsub-arrays are independent, and essentially do not interact. In thiscase, the array states of all such sub-arrays can be searchedsimultaneously, using for each sub-array a list of preferred statesassociated with that particular sub-array. The total dimensionality ofthis search volume can correspond to that of one of the sub-arrays inisolation (i.e., a sub-array with a preferred list representing thelargest dimensionality among those of all of the sub-arrays).

When a total power constraint such as Equation (2) is used, the value ofP₀ can be chosen to guarantee that regardless of what state the array isset to, the radiated power will not exceed certain limits, such asregulatory limits. However, this may be an excessively restrictiveconstraint. Depending on the settings of the various antenna elements,the resulting electromagnetic fields may be partly evanescent in nature.Evanescent fields are characterized by significant storedelectromagnetic energy in the vicinity of the array—which energy may beutilized by RFID tags for operating power—and less propagated energythan would be obtained with the same RF input power and different (moreconventional) array settings.

Compliance with regulatory constraints is in general more dependent onpropagated-energy fields than on stored-energy fields. Thus, when anarray state results in a lower level of propagated fields for a giveninput power, it is generally possible to increase the input power to thearray and still maintain compliance with the applicable regulations. Inthis way, stored-energy field components that may have significantutility for illuminating RFID tags can be made even more effective,while still satisfying regulatory constraints on radiated power.

For a given configuration of antenna elements and given array stateestablished by the controller, the worst-case electromagnetic fieldintensity in the far field can be estimated with calculations orcomputer simulation, or measured in a laboratory setting for arepresentative set of material configurations. This data can be used toset different power levels for each array state so as to maintain themaximum possible excitation fields consistent with applicable regulatoryconstraints. Alternatively, the actual far-field intensity can beactively monitored, and the results used for real-time adjustment oftransmitted power for the same objective.

The far-field intensity from the system for a given input power leveldepends on both the array system and on the material it illuminates.Given an infinite set of possibilities for the material (e.g., contentsand arrangement of cases on a pallet), it is obviously impossible toexhaustively calculate or measure the far-field intensity for allpossible material configurations. However, highly contrived materialconfigurations can be postulated, designed to explore the probabilitiesof far-field coherence of different scattered field components. Forexample, boxes can be constructed or covered with good electricalconductors, and sized and positioned to correspond to the individualantenna elements in the array. By adjusting the displacement betweeneach such box and the antenna element to which it corresponds, the phaseof its contribution to the far field can be varied. From calculations ormeasurements using these contrived material configurations, astatistically sound upper bound on far-field intensities can beprojected for each array state in a given array configuration. Thisinformation can be used directly to set the input power for each arraystate, or to help position auxiliary receive antennas for sensing thefar-field electromagnetic field intensity in real time. When real-timesensing of field intensities is used, the projected upper bound can beused to establish the initial power level, which can then be increasedto a maximum legal level based on the real-time measurement.

Preferably, array states can be switched in coordination with thereader. The criterion for changing array state can be based on the rateat which new tags are being identified within the field of view. Giventhe elapse of a pre-specified interval of time without theidentification of a new tag, the array state can change to the nextstate in the list. Many RFID reader systems already contain logic tocycle through multiple antennas; a different array state can simply beconsidered a different antenna, though utilizing the same physical inputport. A simple modification may be made to an existing reader orreader-controller so that a change of antenna in its usual scanningsequence, whatever criterion is used by the controller for changingantennas, can be executed without physically changing the RF portutilized by the reader. The presence of the array antenna systemessentially presents the reader controller with a new degree of freedomto be exercised in the reader programming.

Another approach useful for more easily achieving back-compatibilitywith existing readers is to accept multiple input ports from the reader,which ports appear to the reader to be independent antenna ports. Eachtime the reader changes antenna ports, this is detected by the arraysystem, which immediately transitions the array to the next state in thestate list, and connects the detected port to the actual array input.

Back-compatibility with existing readers can also be obtained byincorporating a receiver-processor into the array system to monitor theRFID reader's transmissions. By monitoring and detecting features suchas burst durations, signal gaps and frequency hops, the status of thereader within its read cycles can be ascertained, and arraystate-transitions timed to avoid corruption of individual read cycles.

Timing of state transitions can also be assisted by monitoring thedata-reporting function of the RFID reader. A computerized controllertypically accepts output from the reader, and continuously updates astatus list of all tags identified in the current query. This status maybe displayed visually on a computer screen. With any of a variety ofsimple modifications to the programming of the controller, new tagreports can be detected and their frequency calculated; when the rate ofnew tag reads falls below some threshold, the array can be commanded tothe next state.

Finally, though it may not represent an optimal approach in terms ofperformance, the array system can operate asynchronously with thereader, and simply cycle continuously through its available states on apre-determined time line.

EXAMPLE EMBODIMENTS

FIG. 3 shows a simple embodiment of the invention, utilizing a fixed 1:Npower divider 36, and a two-state (0°/180°) phase shifter 38 for eacharray element. Alternatively, the phase shifters can be approximated byswitched-line elements, each having two available line lengths differingfrom each other by approximately one half wavelength at the centerfrequency of the operating band. The array state control block 39 canutilize a pre-stored list of array states, along with the necessarydigital logic to convert each array state to phase shifter commands.Given a strobe signal at the state transition trigger input 37, thearray can progress to the next state in the pre-stored list.

For compatibility with existing readers, such an embodiment can beconnected with a reader through an interface module such as that shownin FIG. 4. This module can exploit existing logic in the reader forswitching between two antenna ports 41, 42. Each of the two RF channelscan be routed through a directional coupler 46, routing a small fractionof the incident RF power to a power detector 49, such as a microwavecrystal detector. The video voltage output of the two detectors can beconverted to voltage levels compatible with standard digital logic suchas TTL, and submitted to a switch logic circuit 43. The switch logiccircuit 43 can control an RF switch 45 selecting one of the two RFchannels 47, 48 for the array RF In/Out port 44. The switch logiccircuit 43 can monitor four logical variables:

-   -   P1=TRUE if incident power detected from Reader Antenna Port 1;    -   P2=TRUE if incident power detected from Reader Antenna Port 2;    -   S1=TRUE if RF switch is set to select Port 1;    -   S2=TRUE if RF switch is set to select Port 2;

Given these definitions, the logic for setting the RF switch can besummarized as follows:S1=P1+ S2S2=P2+S 1  (10)The embodiment of FIG. 4 shows two reader antenna ports 41, 42; theconcept is readily extendable to accommodate a greater number of readerantenna ports. For example, the logic for three ports can be summarizedas following:S1=P1+( S2· S3)S2=P2+( S1· S3)S3=P3+( S1· S2)  (11)As shown, the embodiment of FIG. 3 provides no means for externallyinitializing the array state to the first state in the state list. Thus,a preferred ordering of states can not be readily accommodated. However,the embodiment can be easily modified to automatically reset to theinitial state in the list if no state changes are triggered for somepredetermined duration of time. This function can reside, for example,in the array state control block.

Another embodiment, shown in FIG. 5, includes RF amplifiers 51, 52 toboost both the transmit and receive signals as necessary. The boostedpower can compensate for additional dissipative loss incurred in thepower divider and phase (or line-length) shifters; additionally, if thearray states included in the pre-stored list permit a higher transmitpower than that provided by a standard reader, the RF amplifier 51 inthe transmit path can provide that additional power. If amplification inboth the transmit and receive directions is used, the two propagationdirections can be accommodated with circulators 53 as shown. If thecirculator arrangement is used, then a filter 54 can be used to restrictthe pass-band of one amplifier to a frequency range for which theisolation of the circulators exceeds the combined gain of bothamplifiers.

A more advanced embodiment is shown in FIG. 6, suitable for use inconjunction with a reader controller that can be programmed to directlyutilize and manage the array antenna system. Such a reader controllercan be, for example, implemented with a microcomputer configured withappropriate interface cards for controlling and querying associateddigital devices. Standard RFID reader protocols such as the EPCglobalLow Level Reader Protocol enable such computer-based control of RFIDreaders. A programmable controller can send commands to the reader toestablish various parameters for an RFID read event, trigger the readevent, and download the resulting tag-identification data from thereader. In addition to these tasks, a programmable reader controller canalso access a list of preferred array states, stored within thecontroller or stored remotely, and command the array system to anappropriate state before triggering the RFID read event. Afterdownloading the tag-identification data from the reader, the controllercan update a database associating different array states with differentnumbers of tags successfully read with each state. As described above,this association data can reflect a number of tag ensembles, or beassociated with a particular ensemble of tags. In addition, theembodiment of FIG. 6 shows a variable-gain RF amplifier 61 in thetransmit path, enabling the power utilized for each array state to beoptimized to maximize local electromagnetic field levels within theactive read volume, subject to the applicable regulatory constraints onradiated emissions in the far field.

While FIGS. 5 and 6 show the amplification in path between the array andthe reader, this function could be distributed among the various arrayelements. For example, transmit-receive modules (“T-R modules”) such asthose developed for radar applications could be used. In addition,control functions such as phase and amplitude control could be includedwithin the T-R modules themselves.

EMBODIMENTS Embodiment 1

An antenna system, comprising:

an RF input port, wherein the RF input port receives an input RF signal,wherein the input RF signal is suitable for reading at least one RFIDtag;

a plurality of antennas capable of producing RF fields in a region ofinterest;

a means for producing a corresponding plurality of antenna input signalsfrom the input RF signal, wherein each of the plurality of antenna inputsignals is suitable for reading the at least one RFID tag, wherein whenthe plurality of antenna input signals is inputted to the plurality ofantennas a corresponding plurality of RF fields is produced in theregion of interest;

a state controller, wherein the state controller changes a state of theplurality of antennas by altering the plurality of RF fields, whereinthe state controller produces a first state during a first period oftime and produces a second state during a second period of time,

Embodiment 2

The system according to Embodiment 1, wherein the state controlleralters the plurality RF fields by altering one or more of the following:

a phase of a first at least one of the plurality of antenna inputsignals,

a ratio of amplitudes of a second at least two of the plurality ofantenna input signals,

a radiation pattern of a first at least one of the plurality ofantennas, and

a polarization of a second at least one of the plurality of antennas.

Embodiment 3

The system according to Embodiment 1, wherein the means for producing aplurality of antenna input signals suitable for reading at least one ofthe RFID tag comprises a power divider, wherein the input RF signal isinputted to the power divider and the corresponding plurality of antennainput signals are outputted from the power divider.

Embodiment 4

The system according to Embodiment 1, wherein the state controlleralters the plurality of RF fields by altering a phase of at least one ofthe plurality of antenna input signals.

Embodiment 5

The system according to Embodiment 1, wherein the state controlleralters the plurality of RF fields by altering a ratio of amplitudes ofat least two of the plurality of antenna input signals.

Embodiment 6

The system according to Embodiment 1, wherein the state controlleralters the plurality of RF fields by altering a radiation pattern of atleast one of the plurality of antennas.

Embodiment 7

The system according to Embodiment 1, wherein the state controlleralters the plurality of RF fields by altering a polarization of at leastone of the plurality of antennas.

Embodiment 8

The system according to Embodiment 1, further comprising:

an RFID reader, wherein the RFID reader is coupled to the RF input portand provides the input RF signal.

Embodiment 9

The system according to Embodiment 1, further comprising:

an RF receiver, wherein the RF receiver receives a return RF signal,wherein the return RF signal is due to the plurality of the RF fieldsbeing incident on one or more of the at least one RFID tag.

Embodiment 10

The system according to Embodiment 7, wherein the receiver comprises theplurality of antennas.

Embodiment 11

The system according to Embodiment 9, wherein the received return RFsignal is from the RF input port.

Embodiment 12

The system according to Embodiment 8, further comprising:

an RF receiver, wherein the RF receiver receives a return RF signal,wherein the return RF signal is due to the plurality of the RF fieldsbeing incident on one or more of the at least one RFID tag.

Embodiment 13

The system according to Embodiment 12, wherein the received return RFsignal is outputted from the RF input port to the RFID reader.

Embodiment 14

The system according to Embodiment 9, wherein the receiver comprises asecond plurality of antennas.

Embodiment 15

The system according to Embodiment 14, further comprising:

a second state controller, wherein the second state controller changes areceive state of the second antenna array by altering one or more of thefollowing:

-   -   a phase of a first at least one of a plurality of received        return RF signals as received by the second plurality of        antennas,    -   a ratio of amplitudes of at least two of the plurality of        received return RF signals,    -   a radiation pattern of at least one of the second plurality of        antennas, and    -   a polarization of one or more of the second plurality of        antennas.

Embodiment 16

The system according to Embodiment 15, wherein the second statecontroller produces a first receive state during the first period oftime and produces a second receive state during the second period oftime.

Embodiment 17

The system according to Embodiment 8, further comprising:

one or more RFID tags located in a far-field region of the plurality ofantennas.

Embodiment 18

The system according to Embodiment 17, wherein the far-field region isgreater than or equal to

$\frac{2D^{2}}{\lambda},$where D is the largest dimension of an antenna in a plurality ofantennas normal to radiation direction and λ is the wavelength of theplurality of RF fields.

Embodiment 19

The system according to claim 8, further comprising:

one or more RFID tags located in a radiating near field of the pluralityof antennas.

Embodiment 20

The system according to Embodiment 19, wherein the radiating near fieldis in the range

${{3\lambda} - \frac{2D^{2}}{\lambda}},$where D is the largest dimension of an antenna in the plurality ofantennas normal to radiation direction and λ is the wavelength of theplurality of RF fields.

Embodiment 21

The system according to Embodiment 8, further comprising:

one or more RFID tags located greater than or equal to 3λ away from theplurality of antennas, where λ is the wavelength of the plurality of RFfields.

Embodiment 22

The system according to Embodiment 8, further comprising:

one or more RFID tags located closer than 3λ away from the antennaarray, where λ is the wavelength of the transmitted RF signal.

Embodiment 23

The system according to Embodiment 4, wherein the state controlleralters the phase of the at least one of the plurality of antenna inputsignals via a corresponding plurality of phase shifters, wherein each ofthe plurality of phase shifters receives a one of the plurality ofantenna input signals and outputs the antenna input signal to thecorresponding antenna.

Embodiment 24

The system according to Embodiment 1, wherein each antenna of theplurality of antennas is within

$\frac{\lambda}{2}$of another antenna of the plurality of antennas, where λ is thewavelength of the plurality of RF fields.

Embodiment 25

The system according to Embodiment 1, wherein each antenna of theplurality of antennas is in a range of

$\frac{\lambda}{2}$to λ away from a closest neighbor antenna of the plurality of antennas,where λ is the wavelength of the plurality RF fields.

Embodiment 26

The system according to Embodiment 1, wherein each antenna of theplurality of antennas is greater than λ away from a closest neighborantenna of the plurality of antennas, where λ is the wavelength of theplurality of RF fields.

Embodiment 27

The system according to Embodiment 6, wherein altering the radiationpattern of the at least one of the plurality of antennas comprisesaltering a beam shape of the at least one of the plurality of antennas.

Embodiment 28

The system according to Embodiment 6, wherein altering the radiationpattern of the at least one of the plurality of antennas comprisesaltering a beam pointing angle of the at least one of the plurality ofantennas.

Embodiment 29

The system according to Embodiment 1, wherein at least two of the sameantennas from the plurality of antennas produce one of the plurality ofRF fields in the first state and the second state.

Embodiment 30

The system according to Embodiment 1, wherein the plurality of antennascomprises at least four antennas.

Embodiment 31

A method of reading one or more RFID tags, comprising:

positioning one or more RFID tags in a region of interest;

positioning a plurality of antennas, wherein the plurality of antennasis capable of producing RF fields in the region of interest;

receiving an input RF signal, wherein the input RF signal is suitablefor reading at least one of the one or more RFID tags;

producing a corresponding plurality of antenna input signals from thereceived input RF signal, each of the plurality of antenna input signalssuitable for reading at least one of the one or more RFID tags;

inputting each of the plurality of antenna input signals into acorresponding antenna of the plurality of antennas, such that acorresponding plurality of RF fields are simultaneously produced in theregion of interest from the plurality of antennas while the plurality ofRF fields are in a first state during a first period of time;

receiving a first return RF signal, wherein the first return RF signalis due to the plurality of RF fields produced during the first period oftime being incident on the at least one of the one or more RFID tags;

processing the first return RF signal to determine whether the at leastone of the one or more RFID tags is present in the region of interest;

inputting each of the plurality of antenna input signals into thecorresponding antenna of the plurality of antennas, such that thecorresponding plurality of RF fields are simultaneously produced in theregion of interest from the plurality of antennas while the plurality ofRF fields are in a second state during a second period of time, whereinthe second state is accomplished by altering the plurality of RF fieldsproduced in the region of interest;

receiving a second return RF signal, wherein the second return RF signalis due to the plurality of RF fields produced during the second periodof time being incident on the at least one of the one or more RFID tags;

processing the second return RF signal to determine whether the at leastone of the one or more RFID tags is present in the region of interest.

Embodiment 32

The method according to Embodiment 31, wherein altering at least one ofthe plurality of RF fields produced in the region of interest comprisesone or more of the following:

altering a phase of a first at least one of the plurality of antennainput signals,

-   -   altering a ratio of amplitudes of a second at least two of the        plurality of antenna input signals,    -   altering a radiation pattern of a at least one of the plurality        of antennas, and    -   altering a polarization of a second at least one of the        plurality of antennas.

Embodiment 33

The method according to Embodiment 31, wherein altering at least one ofthe plurality of RF fields produced in the region of interest comprisesaltering a phase of at least one of the plurality of antenna inputsignals.

Embodiment 34

The method according to Embodiment 31, wherein altering at least one ofthe plurality of RF fields produced in the region of interest comprisesaltering a ratio of amplitudes of at least two of the plurality ofantenna input signals.

Embodiment 35

The method according to Embodiment 31, wherein altering at least one ofthe plurality of RF fields produced in the region of interest comprisesaltering a radiation pattern of at least one of the plurality ofantennas.

Embodiment 36

The method according to Embodiment 31, wherein altering at least one ofthe plurality of RF fields produced in the region of interest comprisesaltering a polarization of at least one of the plurality of antennas.

Embodiment 37

The method according to Embodiment 31, further comprising:

a) inputting each of the plurality of antenna input signals into thecorresponding antenna of the plurality of antennas, such that thecorresponding plurality of RF fields are simultaneously produced in theregion of interest from the plurality of antennas while the plurality ofantennas are in at least one additional state during at least oneadditional period of time; and

b) receiving at least one additional return RF signal, wherein the atleast one additional return RF signal is due to the plurality of RFfields produced during the at least one additional period of time beingincident on the at least one of the one or more RFID tags, wherein theat least one additional state is accomplished by altering the pluralityof RF fields produced in the region of interest;

c) processing the at least one additional return RF signal to determinewhether the at least one of the one or more RFID tags is present in theregion of interest.

Embodiment 38

The method according to Embodiment 37, further comprising:

Repeating a, b, and c until a criterion is met.

Embodiment 39

The method according to Embodiment 39, wherein the criterion is all ofthe one or more RFID tags in the region of interest are present.

Embodiment 40

The method according to Embodiment 39, wherein the criterion is allstates have been utilized.

Embodiment 41

The method according to Embodiment 39, wherein the criterion is acertain time period has passed.

Embodiment 42

The method according to Embodiment 39, wherein the criterion is atermination signal is received.

Embodiment 43

The method according to Embodiment 31, wherein receiving the firstreturn RF signal and receiving the second return RF signal isaccomplished via an RF receiver.

Embodiment 44

The method according to Embodiment 43, wherein the RF receiver comprisesthe plurality of receivers.

Embodiment 45

The method according to Embodiment 43, wherein the RF receiver comprisesa second plurality of antennas.

Embodiment 46

The method according to Embodiment 31, wherein receiving the input RFsignal comprises receiving the input RF signal from an RFID reader.

Embodiment 47

The method according to Embodiment 46, further comprising:

inputting the first return RF signal to the RFID reader; and

inputting the second return RF signal to the RFID reader.

Embodiment 48

The system according to Embodiment 4, wherein altering the phase of atleast one of the plurality of antenna input signals is achieved by atleast one variable time-delay.

Embodiment 49

The system according to Embodiment 2, wherein altering the plurality ofRF fields comprises using control elements controlled by analog voltagesthat are generated by analog oscillator circuits.

Embodiment 50

The method according to Embodiment 31, wherein possible states areordered to place states having a statistically higher probability ofsuccessfully reading tags earlier in the ordering.

Embodiment 51

The method according to Embodiment 31, wherein possible states areordered to place array states with a proven history of successfullyreading tags earlier in the ordering.

Embodiment 52

The method according to Embodiment 50, wherein the probability ofsuccessfully reading tags applies to a specific ensemble of tags in afixed configuration.

Embodiment 53

The method according to Embodiment 50, wherein the proven history ofsuccessfully reading tags applies to a specific ensemble of tags in afixed configuration.

Embodiment 54

The method according to Embodiment 50, wherein the probability ofsuccessfully reading tags applies to a category of ensembles.

Embodiment 55

The method according to Embodiment 50, wherein the proven history ofsuccessfully reading tags applies to a category of ensembles.

Embodiment 56

The method according to Embodiment 31, wherein possible states aredefined as a set of all possible combinations of digital commands tovarious electronic control elements.

Embodiment 57

The method according to Embodiment 56, wherein the states have beenordered so that all possible permutations of the most-significant bitsof various digital commands are cycled through first.

Embodiment 58

The method according to Embodiment 31, wherein data from the RFID readerand/or a controller of the RFID reader is monitored to detect reports ofnewly identified tags, wherein an interval between successive new tagidentifications is calculated from detections, and wherein the state ischanged when the interval between new tag identifications is longer thana specified threshold.

Embodiment 59

The method according to Embodiment 31, wherein transmissions of the RFIDreader are monitored and processed, and wherein signal features from thetransmissions such as burst durations, signal gaps, and frequency hopsare used to ascertain the status of the reader with respect to thereader's read cycles.

Embodiment 60

The method according to Embodiment 59, wherein the signal features areutilized to time state transitions so as to avoid corruption ofindividual read cycles.

Consistent with the various descriptions and discussions within thisdocument, numerous variations on these example embodiments are possible,while remaining within the spirit and intent of the invention.

Though many specific design details have been omitted from thedescriptions and embodiments disclosed herein, in the hands ofpractitioners skilled in the relevant arts the information disclosed isquite sufficient to enable the engineering and construction ofpractical, working devices.

In specific embodiments, a variety of techniques can be used to controland operate the subject device. Control elements are controlled byanalog voltages that are generated by digitally controlled circuits suchas digital-to-analog converters. Active electronics modules areassociated with one or more of the individual array elements. One ormore antennas can transmit signals that have been digitally encoded andreconstructed, for example with analog-to-digital converters anddigital-to-analog converters. One or more control functions, such asphase and amplitude, an be implemented numerically at a point where thesignals are still in digital format. The device can incorporate areceiver and processor to monitor an RFID reader's transmissions, anduse signal features such as burst durations, signal gaps and frequencyhops to ascertain the status of the reader with respect to its readcycles, and use this information to time array state transitions so asto avoid corruption of individual read cycles. The device canincorporate a data link from a computer or controller that is receivingdata from the RFID, along with programming to calculate the intervalsbetween new reports of tags successfully identified, and compare one ormore intervals to a specified threshold, and command the array to a newstated when some determined number of intervals exceeds the specifiedthreshold.

An embodiment of the subject device can also incorporate an interfacemodule providing multiple antenna ports, where the interface moduledetects the incident radio-frequency power in each of the antenna ports,and a radio-frequency signal switch that selectively connects theradio-frequency signal port of the antenna array system to any one ofthe various antenna ports, together with logic to interpret thedetections of incident radio-frequency power, together with means tocontrol said radio-frequency signal switch according to this logic. Thelogic can determine when an external radio-frequency signal source suchas a radio-frequency-identification reader has changed from exciting oneof the antenna ports to exciting a different antenna port. In responseto this change, the interface module can trigger the antenna arraysystem to transition from the current array state to the next arraystate in the defined sequence of array states, and can also connect theradio-frequency signal port to the newly excited antenna port via saidradio-frequency signal switch.

Embodiments can have separate radio-frequency signal channels fortransmitted and received signals. Transmitted signals can be defined assignals input at the radio-frequency signal port and conveyed by theantenna array system to its various antenna elements and receivedsignals can be defined as signals incident on one or all of the variousantenna elements and conveyed to the radio-frequency signal port by theantenna array system. Radio-frequency amplification may be incorporatedinto either or both said radio-frequency signal channels to boost thepower level of transmitted signals, received signals or both transmittedand received signals.

Embodiments can allow resetting of the array to the initial state in itsdefined sequence of states. The stepped through the entire definedsequence of array states, wherein the reset operation may be in responseto a command accepted as an input to the antenna array system, or may beexecuted automatically by the antenna array system if no transitions inarray states are executed for some predetermined interval of time. Thedefined sequence of array states can be ordered so the antenna arraysystem steps through preferred array states earlier in said definedsequence and through less preferred array states later in the definedsequence, wherein the degree of preference for a given state isdetermined by measured, calculated, predicted or expected superiority oradequacy in performance for a given application.

In an embodiment, one or more elements of the ensemble of antennas canhave electrically or electronically controllable properties such as beampointing angle, beam shape, or electromagnetic polarization, and eacharray state of said antenna array system can have a unique combinationof the transfer functions and controllable properties of individualelements within said ensemble of antennas.

In specific embodiments, detectors can be used to measure or monitor theradiated electromagnetic field at strategic locations in the vicinity ofthe array system, together with means to adjust the input power to saidarray system for each array state so as to maintain a defined constrainton the radiated field as measured or monitored at the strategiclocations can be provided.

Means to adjust the gain of said radio-frequency amplification means soas to maintain a defined constraint on the radiated field as measured ormonitored at the strategic locations can be provided.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

What is claimed:
 1. An antenna system, comprising: an RF input port,wherein the RF input port receives an input RF signal, wherein the inputRF signal is suitable for reading at least two RFID tags; a plurality ofantennas, wherein each antenna of the plurality of antennas isconfigured to produce an RF field in a volume of interest when anantenna input signal is inputted to the each antenna of the plurality ofantennas, wherein one or more antenna of the plurality of antennas isconfigured to produce an RF field at any position in the volume;circuitry adapted to produce a corresponding plurality of antenna inputsignals from the input RF signal, wherein each of the antenna inputsignal the plurality of antenna input signals is suitable for readingthe at least two RFID tags, wherein when the plurality of antenna inputsignals is simultaneously inputted to the plurality of antennas acorresponding plurality of RF fields is simultaneously produced in thevolume of interest; a state controller, wherein the state controllerchanges a state of the plurality of antennas by altering the pluralityof RF fields, wherein the state controller produces a first state duringa first period of time and produces a second state during a secondperiod of time, wherein the state controller alters the plurality RFfields by altering one or more of the following: a phase of a first atleast one of the plurality of antenna input signals, wherein alteringthe phase of the first at least one of the plurality of antenna inputsignals is independent of altering a phase of any other of the pluralityof antenna input signals; a ratio of amplitudes of a first at least twoof the plurality of antenna input signals; a radiation pattern of afirst at least one of the plurality of antennas; and a polarization of asecond at least one of the plurality of antennas.
 2. The systemaccording to claim 1, wherein the circuitry adapted to produce theplurality of antenna input signals comprises a power divider, whereinthe input RF signal is inputted to the power divider and thecorresponding plurality of antenna input signals are outputted from thepower divider.
 3. The system according to claim 1, wherein the statecontroller alters the plurality of RF fields by altering a phase of atleast one of the plurality of antenna input signals.
 4. The systemaccording to claim 1, wherein the state controller alters the pluralityof RF fields by altering a ratio of amplitudes of at least two of theplurality of antenna input signals.
 5. The system according to claim 1,wherein the state controller alters the plurality of RF fields byaltering a radiation pattern of at least one of the plurality ofantennas.
 6. The system according to claim 1, wherein the statecontroller alters the plurality of RF fields by altering a polarizationof at least one of the plurality of antennas.
 7. The system according toclaim 1, further comprising: an RFID reader, wherein the RFID reader iscoupled to the RF input port and provides the input RF signal.
 8. Thesystem according to claim 1, further comprising: an RF receiver, whereinthe RF receiver receives a receive RF signal, wherein the receive RFsignal is due to the plurality of the RF fields being incident on one ormore of the at least two RFID tags.
 9. The system according to claim 6,wherein the receiver comprises the plurality of antennas.
 10. The systemaccording to claim 8, wherein the received receive RF signal isoutputted from the RF input port.
 11. The system according to claim 7,further comprising: an RF receiver, wherein the RF receiver receives areceive RF signal, wherein the receive RF signal is due to the pluralityof the RF fields being incident on one or more of the at least two RFIDtags.
 12. The system according to claim 11, wherein the received receiveRF signal is outputted from the RF input port to the RFID reader. 13.The system according to claim 8, wherein the receiver comprises a secondplurality of antennas.
 14. The system according to claim 13, furthercomprising: a second state controller, wherein the second statecontroller changes a receive state of the second plurality of antennasby altering one or more of the following: a phase of a first at leastone of a plurality of received receive RF signals as received by thesecond plurality of antennas, a ratio of amplitudes of at least two ofthe plurality of received receive RF signals, a radiation pattern of atleast one of the second plurality of antennas, and a polarization of oneor more of the second plurality of antennas.
 15. The system according toclaim 14, wherein the second state controller produces a first receivestate during the first period of time and produces a second receivestate during the second period of time.
 16. The system according toclaim 7, further comprising: one or more RFID tags located in afar-field region of the plurality of antennas.
 17. The system accordingto claim 16, wherein the far-field region is greater than or equal to$\frac{2D^{2}}{\lambda},$ where D is the largest dimension of an antennain a plurality of antennas normal to radiation direction and λ is thewavelength of the plurality of RF fields.
 18. The system according toclaim 7, further comprising: one or more RFID tags located in aradiating near field of the plurality of antennas.
 19. A method ofreading two or more RFID tags, comprising: positioning two or more RFIDtags in a volume of interest; positioning a plurality of antennas,wherein each antenna of the plurality of antennas is configured toproduce an RF field in the volume of interest when an antenna inputsignal is inputted to the each antenna of the plurality of antennas,wherein one or more antenna of the plurality of antennas is configuredto produce an RF field at any position in the volume of interest;receiving an input RF signal, wherein the input RF signal is suitablefor reading at least one of the two or more RFID tags; producing acorresponding plurality of antenna input signals from the received inputRF signal, each of the plurality of antenna input signals suitable forreading at least one of the one or more RFID tags; inputting each of theplurality of antenna input signals into a corresponding antenna of theplurality of antennas while the plurality of antennas is in a firststate during a first period of time, such that a corresponding pluralityof RF fields are simultaneously produced in the volume of interest fromthe plurality of antennas; receiving a first receive RF signal, whereinthe first receive RF signal is due to the first plurality of RF fieldsproduced during the first period of time being incident on the at leastone of the two or more RFID tags; processing the first receive RF signalto determine whether the at least one of the two or more RFID tags ispresent in the volume of interest; inputting each of the plurality ofantenna input signals into the corresponding antenna of the plurality ofantennas while the plurality of RF fields are in a second state during asecond period of time, such that a corresponding second plurality of RFfields is simultaneously produced in the volume of interest from theplurality of antennas, wherein the second plurality of RF fieldsproduced in the volume of interest is altered compared with the firstplurality of RF fields produced in the volume of interest; receiving asecond receive RF signal, wherein the second receive RF signal is due tothe plurality of RF fields produced during the second period of timebeing incident on the at least one of the two or more RFID tags;processing the second receive RF signal to determine whether the atleast one of the two or more RFID tags is present in the volume ofinterest.
 20. A non-transitory, computer readable medium containing aset of instructions that when executed cause a computer to perform amethod of reading two or more RFID tags, wherein the method comprises:positioning two or more RFID tags in a volume of interest; positioning aplurality of antennas, wherein each antenna of the plurality of antennasis configured to produce an RF field in the volume of interest when anantenna input signal is inputted to the each antenna of the plurality ofantennas, wherein one or more antenna of the plurality of antennas isconfigured to produce an RF field at any position in the volume ofinterest; receiving an input RF signal, wherein the input RF signal issuitable for reading at least one of the two or more RFID tags;producing a corresponding plurality of antenna input signals from thereceived input RF signal, each of the plurality of antenna input signalssuitable for reading the at least one of the two or more RFID tags;inputting each of the plurality of antenna input signals into acorresponding antenna of the plurality of antennas while the pluralityof antennas is in a first state during a first period of time, such thata corresponding first plurality of RF fields is simultaneously producedin the volume of interest from the plurality of antennas; receiving afirst receive RF signal, wherein the first receive RF signal is due tothe first plurality of RF fields produced during the first period oftime being incident on the at least one of the two or more RFID tags;processing the first receive RF signal to determine whether the at leastone of the two or more RFID tags is present in the volume of interest;inputting each of the plurality of antenna input signals into thecorresponding antenna of the plurality of antennas while the pluralityof antennas is in a second state during a second period of time, suchthat a corresponding second plurality of RF fields is simultaneouslyproduced in the volume of interest from the plurality of antennas,wherein the second plurality of RF fields produced in the region ofinterest is altered compared with the first plurality of RF fieldsproduced in the volume of interest; receiving a second receive RFsignal, wherein the second receive RF signal is due to the plurality ofRF fields produced during the second period of time being incident onthe at least one of the two or more RFID tags; processing the secondreceive RF signal to determine whether the at least one of the two ormore RFID tags is present in the volume of interest, wherein the secondplurality of RF fields produced in the volume of interest is altered ascompared to the first plurality of RF fields produced in the volume ofinterest via one or more of the following: altering a phase of a firstat least one of the plurality of antenna input signals, wherein alteringthe phase of the first at least one of the plurality of antenna inputsignals is independent of altering a phase of any other of the pluralityof antenna input signals, altering a ratio of amplitudes of a first atleast two of the plurality of antenna input signals, altering aradiation pattern of a first at least one of the plurality of antennas,and altering a polarization of a second at least one of the plurality ofantennas.